Time-of-flight mass spectrometry with pulsed glow discharge for direct determination of volatile organic compounds in air, nitrogen and argon. Volatile organic compounds ionization processes

Abstract

Determination of volatile organic compounds (VOCs) in various gases, including atmospheric and exhaled human air, is required to solve a wide range of environmental problems, control the composition of various gases and is increasingly used for diagnosis of various diseases. Lately, methods of soft ionization with minimal fragmentation of the components have been rapidly developed. In particular, our research group is developing an approach to direct analysis of mixtures of VOCs using time-of-flight pulsed glow discharge mass spectrometry. Previously, the effects of different gases and gas mixtures on ionization processes were not compared. Therefore, the ionization mechanisms of VOCs in argon, nitrogen, and air were investigated in the present work. Toluene, p-xylene, chlorobenzene and 1,2,4-trimethylbenzene were chosen as the model VOCs. Optimization of microsecond pulsed glow discharge parameters (period and duration of the discharge pulse, repelling pulse delay time and pressure in the discharge cell) for each compound and a gas mixture of several VOCs was carried out. The predominant ionization mechanisms are the formation of a VOC molecular ion by Penning ionization and the proton transfer reaction; their influence being different for various gases. It is shown that the use of argon even with a small addition of water leads to the predominance of the proton transfer reaction, whereas in nitrogen and air the Penning ionization predominates. The maximum VOC intensities were achieved in air, and the developed approach